升压转换::可选为bool [英] Conversion of boost::optional to bool

问题描述

我如何prevent这个code的最后一行进行编译？

How I can prevent the last line of this code from compiling?

#include <boost/optional.hpp>

int main()
{
    typedef boost::optional<int> int_opt;
    int_opt opt = 0;
    bool x = opt;  // <- I do not want this to compile
}

最后一行不检查选择的包含int值，而是编译如下类型转换为bool，并且似乎没有什么用户意。

The last line doesn't examine opt's contained int value, but instead compiles as a type conversion to bool, and doesn't seem to be what the user intended.

在安全布尔成语似乎是与此有关？

The safe bool idiom seems to be relevant here?

推荐答案

的boost ::可选的的全部要点是使code这样的：

The whole point of boost::optional is to enable code like this:

void func(boost::optional<int> optionalArg)
{
    if (optionalArg) {
       doSomething(*optionalArg);
    }
}

所以隐式转换为布尔是一个功能，而不是应该从编制pvented $ P $。

So the implicit conversion to bool is a feature, and should not be prevented from compiling.

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